The efficient functioning of the shoulder joints is important to the well-being and mobility of the human body. Each shoulder joint includes the upper portion of the humerus, which terminates in an offset bony neck surmounted by a ball-headed portion known as the humeral head. The humeral head rotates within a socket, known as the glenoid fossa, in the scapula to complete the shoulder joint. Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining of the glenoid fossa so that the ball of the humerus and the scapula rub together, causing pain and further erosion. Bone erosion may cause the bones themselves to attempt to compensate for the erosion which may result in the bone becoming deformed. This misshapen joint may cause pain and may eventually cease to function altogether.
Operations to replace the shoulder joint with an artificial implant are well-known and widely practiced. Generally, the shoulder prosthesis will be formed of two components, namely: a glenoid, or socket, component which lines the glenoid fossa, and a humeral, or stem, component which includes a weight-bearing ball and replaces the humeral head. (Each of these components may be made up of multiple subassemblies.) Alternately, a reverse shoulder prosthesis has a ball as the glenoid component and a socket as the humeral component, but the following description presumes a standard, not reverse, shoulder prosthesis arrangement. During the surgical procedure for implanting the shoulder prosthesis, the remaining cartilage or damaged tissue is removed from the glenoid fossa using a reamer such that the native glenoid fossa will accommodate the outer surface of the glenoid component of the shoulder prosthesis. The glenoid component of the prosthesis can then be inserted into the prepared glenoid fossa. Generally, fixing means such as screws and/or bone cement may be used to hold the glenoid component in the glenoid fossa. There is also generally an implant stem provided to the glenoid implant, the stem being inserted into a prepared cavity in the glenoid fossa to anchor the glenoid implant. The use of additional fixing means and anchor(s) helps to provide stability after the prosthesis has been inserted. In some modern prosthesis, the glenoid component may be coated on its external surface with a bone growth promoting substance which will encourage bone ingrowth which also helps to hold the glenoid component in place. The humeral head also is removed during the surgical procedure, and the humerus shaft hollowed out using reamers and rasps to accept the humeral component of the prosthesis. The stem portion of the prosthesis is inserted into the humerus and secured therein to complete the shoulder joint replacement.
In order to strive toward desired performance of the combined glenoid and humeral shoulder prosthesis components, the glenoid portion should be properly positioned upon the glenoid fossa (among other considerations). The glenoid portion positioning is particularly important since incorrect positioning of the glenoid component can lead to the prosthetic shoulder joint suffering from dislocations, a decreased range of motion, and possibly eventual loosening and/or failure of one or both components of the joint.
Generally, the normal glenoid retroversion of a given patient may fall within a range of approximately 20° (5° of anteversion and 15° of retroversion). (The version of the glenoid is defined as the angle between the plane of the glenoid fossa to the plane of the scapula body.) In the pathologic state, glenoid bone loss may result in a much larger range of version angles.
One goal of shoulder surgery may be to modify the pathologic bone to correct pathologic version to be within the normal range or the normal version of the patient's native anatomy before the bone loss occurred. During surgery, and particularly minimally invasive procedures, the plane of the scapula may be difficult or impossible to determine by direct visual inspection, resulting in the need for assistive devices or methods to define both the pathologic version present at the time of surgery and the intended correction angle.
It is generally believed that there is a preferred orientation for the glenoid component to provide a full range of motion and to minimize the risk of dislocation or other mechanical component failure. Some example orientations of the glenoid prosthesis relative to the glenoid face are about 5° of anteversion to about 15° of retroversion; average version is about 1-2° of retroversion. This broadly replicates the natural angle of the glenoid. However, the specific angular orientation of the glenoid portion varies from patient to patient.
With a view to overcoming these disadvantages, some arrangements have been recently suggested in which a three-dimensional intraoperative computer imaging surgical navigation system is used to render a model of the patient's bone structure. This model is displayed on a computer screen and the user is provided with intraoperative three-dimensional information as to the desired positioning of the instruments and the glenoid component 216 (or any component, depending on the subject patient tissue) of the prosthetic implant. However, surgical navigation arrangements of this type are not wholly satisfactory since they generally use only a low number of measured landmark points to register the patient's anatomy and to specify the angle of the prosthetic implant component (e.g., a glenoid component 216), which may not provide the desired level of accuracy. Further, the information provided by such systems may be difficult to interpret and may even provide the user with a false sense of security. Moreover, these systems are generally expensive to install and operate and also have high user training costs. Various proposals for trial prosthetic joint components and assistive instruments have been made in an attempt to overcome the problems associated with accurately locating the glenoid portion of the prosthetic implant. While these trial systems and instruments may help with checking whether the selected position is correct, they are not well-suited to specify the correct position initially, and thus there still is user desire for a system which may assist a user in placement of prosthetic implant component in a prepared native tissue site.